1. Field of the Invention
The present invention relates to half metallic ferromagnetic chromium dioxide (CrO2) in substantially pure form. The present invention further relates to composites of chromium dioxide and other oxides of chromium namely, CrO2/Cr2O3 and CrO2/Cr2O5. The present invention further relates to a process for manufacturing said substantially pure chromium dioxide and composites of chromium dioxide and other oxides. Chromium dioxide is a well known material used in magnetic recording applications. Apart from this, CrO2 and the composites of CrO2 with other oxides of chromium have wide application as a magnetoresistive and spintronic material.
2. Related Art of the Invention
Pure CrO2: Chromium dioxide (CrO2) is a metallic, room temperature ferromagnet with Curie temperature (Tc) around 393 K. It has been widely used as a particulate media in magnetic recording applications since long. B. L Chamberland has reviewed in 1977 “The chemical and physical properties of CrO2 and tetravalent chromium oxide derivatives” in CRC Critical Rev. in Solid State and Mater. Sci. 7, 1 (1977).
CrO2 is also well known to be a half metallic ferromagnet. Half metallic ferromagnets are defined as ferromagnets in which conduction electrons are fully spin polarized at low temperatures where spin disorder is minimum (see “New class of materials: half metallic ferromagnets” R. A. Groot et. al, Phy. Rev. Lett 50, 2024, 1983). It is desirable to maintain the spin polarization near room temperatures so that devices based on spin polarised current could be realized. The phenomenon of spin polarized conduction has given rise to an upcoming field of spintronics wherein devices are based on spin polarized current. In 2001, Ji et. al, have observed, through Andreev reflection measurements, the maximum spin polarization close to 100% in CrO2, vis a vis other materials showing half metallicity. See “Determination of the spin polarization of half metallic CrO2 by point contact Andreev reflection”, Phy. Rev. Lett., Y. ji et. al., 86, 5585, 2001.
Polycrstalline CrO2 is now well established to be a magnetoresistive and spintronic material. Here the magnetoresistance (MR) is defined as ΔR/Ro=(RH−Ro)/Ro, where Ro and RH are the electrical resistance in zero field and in external magnetic field (H), respectively. Materials displaying negative MR greater that a few percent are used in various devices. Related prior art can be seen in U.S. Pat. No. 5,856,008.
Interestingly, CrO2 is known to show the metallic conductivity in single crystals and epitaxially grown thin films. However, activated behavior is seen in polycrystalline CrO2 that is believed to be arising from hopping/tunneling of the charge carrier across grain boundaries in polycrystals. This mechanism of ‘spin polarized tunneling’ of charge carriers across the grain boundaries which exist in polycrystalline samples were first proposed by Hwang et. al in their paper entitled “Spin polarized intergrain tunneling in La2/3Sr1/3MnO3, Phys. Rev. Lett. 77, 2041 (1996). J M D Coey et. al, have related this mechanism of conduction to the origin of large magnetoresistance exhibited by polycrystalline CrO2 in their paper entitled “Magnetoresistance of Chromium dioxide powder compacts, Phy. Rev. Lett. 80, 3815, 1998”. One area in which half metallic ferromagnets have tremendous device application is Magnetic Tunnel Junctions. (Warren E Picket and Jagdeesh S Moodera, “Half-metallic Magnets” Physics Today, May 2001). The half-metallic ferromagnets, as a magnetoresistive material have been employed in various magneto-electronic applications.
The utility of above-mentioned devices is significant when substantial MR is found near room temperatures. In case of polycrystalline CrO2, it is found that MR is maximum at lowest temperatures of the order of 5K and is known to decrease rapidly with increasing temperatures. For instance Coey et. al have reported MR of the order of 0.1% near room temperature in their paper entitled “Magnetoresistance of Chromium dioxide powder compacts, Phy. Rev. Lett. 80, 3815, 1998”. There are several factors that may cause the loss of spin polarization. The presence of even a small amount of impurity phase not only affects the ferromagnetic properties but also interferes with the phenomenon of half metallicity/spin polarization severely. This results in the loss of spin polarization and thus brings down the efficiency of the concerned device, based on spin-polarized current.
Since CrO2 is a material of industrial importance, there have been a large number of patents and papers on various preparation methods and intricacies involved for preparing CrO2. The related prior art is presented in Table 1. There are three important factors related to synthesis of CrO2 (i) Taking CrO3 as starting material, CrO2 is not known to form in ambient pressure. (ii) Pressure temperature phase diagram is highly interlinked resulting in mixed phase compounds. (iii) It is known that once formed, sintering of CrO2 is difficult since it is a metastable phase and easily converts to Cr2O3 even at modest temperatures of 200° C. (See L. Ranno, A. Barry and J. M. D. Coey, J. Appl. Phys., 81, 5774 (1997)). This is an important issue related with the fabrication and reproducibility factor of devices based on CrO2.
TABLE 1U.S. Pat. No.Starting MaterialTemp. PressureProductRemarks2923685 (1960)CrO3, H2O, Na2SO4450° C., 1000 AtmCrO2*3423320 (1969)KCr3O8, H2O,2600 AtmCrO2needle like FM3449073 (1969)CrO3, Cr2O3, Na2Cr2O7 850 AtmCrO2fine grain particles4428852 (1984)preheated hydratedelevated pressure/CrO2*Chromium oxidecontinuous process3117093 (1964)CrxOy 2y/x is 4–5.550–3000 AtmCrO2*3493338 (1970))CrO3, NO, O2225, 325° C. no pressureCrO2 94% + CrO3, 5%*5856008 (1999)CrO3520° C. 35000 barCrO2*CrO2 coated with Cr2O3***Relevant to production of CrO2;**relevant to production of CrO2/Cr2O3 Composite.
Table 1 shows that    (a) CrO3 (chromium VI oxide) has been used as a starting material. CrO2 can be prepared by thermal decomposition of CrO3 and mixed chromium oxides.    (b) There exist a very narrow window of temperature and pressure in which many other oxides of chromium stabilize, including CrO2, and the phase boundary between these oxides is very fuzzy. Consequently a little variation in preparation condition results in mixed phase or impure compounds Chamberland has discussed this aspect by showing it in FIGS. 1 and 2 on page 3 of his review (ibid.).    (c) The difficulties of accurately measuring and controlling pressure at elevated temperatures, along with the fact that it requires expensive high-pressure assemblies leading to high production cost are the main drawbacks of above-mentioned preparative methods. It is desirable to have a preparative method, which does not need pressure as controlling parameter.    (d) The last three U.S. patents in the above table form relevant prior art to the present invention and will be discussed after describing the present invention in detail.
A ferromagnetic sample is characterized by its saturation magnetization Ms at 0 K and Curie temperature Tc. The theoretical value for saturation magnetization for CrO2 is about 135 emu/gm. The best-reported value for the saturation magnetization for polycrystalline samples range from 75-87 emu/gm as reported in earlier patents (Table 2). The single crystals have shown value of the order of 108 emu/gm. The best values of Ms for polycrystalline CrO2 supplied by DuPont is from 87-110 emu/g as given in “Spin phonon coupling in rod shaped half metallic CrO2 ultra fine particles: a magnetic Raman scattering study, T Yu et al., J. Phys. Condens. Matter 15, L213, 2003 and “Junction like magnetoresistance of intergranular tunneling in field aligned chromium dioxide powders”, Jianbiai Dai and Jinke Tang, Phy. Rev. B, 63, 054434 (2001).
Since the saturation magnetization value (Ms) is an important criterion for a pure ferromagnetic material, and is a test for comparing various processes, some Ms values from literature for CrO2 are given in Table 2.
TABLE 2Saturation Magnetization Values for CrO2 asreported in literatureSaturation Magnetization (Ms)Reference(emu/gm)Theoretical Value of~135*MsU.S. Pat. No. 474797475–78(polycrystalline)U.S. Pat. No. 349333824–84 (including modifier)(polycrystal)U.S. Pat. No. 348685178–87(polycrystal)U.S. Pat. No. 345177121–35(polycrystal)U.S. Pat. No. 292368338–66 (with modifier)(polycrystal)Poly crystal73–75Chamberland (1977)Single crystal108(Chamberland 1977)Polycrystals~100–110 emu/gsupplied by DuPont*Note: For calculating the theoretical value of Ms, density of the CrO2 is taken as 4.8 g/cm3 However the density calculated from X-Ray measurements is found to be 4.89 g/cm3 whereas observed density is of the order of 4.8 g/cm3. This point is discussed in the review article on CrO2 by Chamberland, Page 10
Composites of CrO2 and Cr2O3: An important aspect related with CrO2/Cr2O3 composites is that it is also a magnetoresistive material. The most important aspect of composites of chromium dioxide and chromium sesquioxide (CrO2/Cr2O3) is that they show an enhancement of magnetoresistance ratio over pure polycrystalline chromium dioxide at low temperatures. It is known that the percentage magnetoresistance is increased when insulating Cr2O3 is added to metallic CrO2. However, composites having high magnetoresistance near room temperature have not been reported which is more significant from the device application point of view. Moreover, the known composites are in the cold pressed form due to the reason that CrO2 once formed, is known to be a metastable phase which degrades at temperatures as moderate as 200° C. Due to this reason neither pure CrO2 nor its composites can be sintered to make hard pellets for practical usage. Hard pellets are made by putting chemical binder etc. This difficulty exists for bulk transport measurements for which sintered forms of the composite is desirable.
Composites prepared by known methods lack homogeneity as well as reproducibility factor which is important for all practical purposes of using composites as a magnetoresistive material in device applications. Generally a CrO2/Cr2O3 composite may be obtained by (i) annealing pure CrO2 in oxygen at elevated temperatures preferable in the range 350 to 500 C (U.S. Pat. No. 5,856,008) (ii) mixing of pure CrO2 in pure Cr2O3 to obtain a composite in desirable ratio (See J M D Coey et al Phy. Rev. Lett. 80, 3815, 1998). Both annealing process and mixing process have their own disadvantage. For instance as taught in U.S. Pat. No. 5,856,008, annealing of CrO2 above a certain temperature results in Cr2O3 layer of thickness that inhibit intergrain tunneling and therefore reduce the magnetoresistance. This puts the upper limit on the mass fraction of Cr2O3 in CrO2. Since it is known that the % MR can be tuned depending on the mass fraction of Cr2O3 in CrO2, it is desirable to have a process through which mass fraction of Cr2O3 can be easily controlled. Similarly a composite obtained by mixing CrO2 and Cr2O3 in desired ratio cannot be obtained in sintered form, besides the final MR value depends on the grain size and porosity of the sample, which may vary with grinding time, and pressure at which the samples were cold pressed. Thus known methods do not allow systematic control of homogeneous proportion of Cr2O3 in a composite. Additionally, in the known methods, varying oxygen pressure controls the proportion of Cr2O3 and a slight variation in experimental condition may adversely affect the reproducibility of physical properties.
Composites of CrO2 and Cr2O5: Cr2O5 is another oxide of Cr which is insulating. The product Cr2O5 is a well known compound and is well documented in literature. However the composites of CrO2 with Cr2O5 have not been studied in terms of % MR.